Currently, lithium cobalt oxide, lithium manganese oxide, nickel-cobalt-manganese ternary material and lithium iron manganese phosphate have been successfully applied to positive electrode materials for lithium-ion batteries, in which lithium cobalt oxide has essentially occupied the market of small-sized lithium-ion batteries, but it cannot serve as positive electrode materials for lithium-ion power batteries due to its high price, low performance and high hidden dangers. Thus, in recent years the other three kinds of materials have attracted much attention as positive electrode materials for lithium-ion power batteries. However, the above three kinds of positive electrode materials have an actual specific capacity generally less than 150 mAh/g. Consequently, to develop lithium-ion power batteries having broad prospects of application, there is an urgent need to develop a novel positive electrode material with a high specific capacity, a low cost and a high safety. Among the novel positive electrode materials that have been developed, lithium-rich manganese-based positive electrode materials with a specific discharge capacity up to 300 mAh/g have received great attention from the researchers.
In terms of current studies, lithium-rich manganese-based positive electrode materials have technical problems such as low initial coulombic efficiency, poor rate performance and poor cycle performance, which hamper its large-scale application. Studies show that the coulombic efficiency of lithium-rich manganese-based positive electrode materials can be effectively improved by surface coating. A method for surface coating a lithium-rich positive electrode material using MnO2 is disclosed, for example, in Chinese Patent Publication No. CN101562245A, which reduces the initial irreversible capacity loss of the material and improves the cycle performance of the material at a high rate, but the modified material still has a relatively low discharge capacity. In Chinese Patent Publication No. CN102738458A, oxides or phosphates of the elements such as Al, Ce, Mn, Ru, Y, Ni, Co and the like are used as a coating layer of a lithium-rich manganese-based positive electrode material, which increases the initial coulombic efficiency and improves cycle performance and rate performance, but this modified method decreases the discharge voltage plateau of the lithium-rich manganese-based positive electrode material, resulting in a decrease in the energy density of the lithium-rich positive electrode material. Chinese Patent Publication No. CN102694164A discloses a lithium-rich manganese-based positive electrode material doped with nitrogen or carbon on the surface, which improves the cycle performance of the lithium-rich manganese-based positive electrode material, but this method does not solve the problem about low initial coulombic efficiency.
Recently, a non-patent literature (J. Electrochem. Soc. 153, A1186-A1192, (2006)) proposes a method for treating a surface of a lithium-rich manganese-based positive electrode material with strong acids, and the results indicate that chemical activation with the strong acid treatment is an effective means for improving the initial efficiency. However, during the process of acid treatment, though lithium vacancies and oxygen vacancies are formed on the particle surface, the surface structure of the material is severely damaged and part of H protons are exchanged with Li ions, thus causing significant decreases in the cycle performance and the rate performance of the material.